There are a variety of known active interrogation systems directing some type of energy at a target to attempt to identify materials in the target. Laser induced active sensing techniques such as fluorescence, Raman, and multi/hyper spectral analysis generally have limited ability to penetrate into the bulk of target materials. Another system uses nuclear activation via a neutron beam to activate substances, which creates significant safety issues based on radiation.
They are usually employed to interrogate the outside surface of a target material. Some interrogation systems use energy having TeraHertz frequencies, which are limited to very short range applications, such as less than one meter. Nuclear quadrupole magnetic resonance imaging systems can detect explosives, but are subject to defeat by simple RF shielding. While standoff detection systems above may have some practical utility, each has certain limitations and/or safety issues in various applications. The most sensitive techniques are well suited to trace or residue detection on an exterior surface. Passive EO techniques have also been employed for residue detection. Raman inspection systems are sensitive and can in principle operate at larger stand-off ranges, however, they are still limited to detection of the residues only and not bulk materials detection inside a container or targets of interest typically in a container or behind a barrier.
Conventional X-ray sensors can penetrate containers for indirect interrogation of suspect explosive and contraband materials concealed within. One type of X-ray backscatter system utilizes broad, incoherent, CW X-ray energy to classify target materials by density maps/differences. Other X-ray sensor systems operate in a transmission mode, and yet other standoff x-ray/γ-ray sensors utilize Compton backscattering and/or induced fluorescence to detect materials. In most examples, the results are shown in a two-dimensional output, such as a computer display, a radiograph/photograph, or similar. All information related to a profile of the target material as a function of its thickness or depth is lost, having been collapsed into the two-dimensional output.
In some cases, the constraint of a two-dimensional output can defeat the purpose of the X-ray examination. When the target material is enclosed within a container, returns derived from the container walls and atmospheric scattering contributions are collected as components of the two-dimensional output. In that case, interference from the container wall and atmosphere at least obscures the output, and it may dominate the output, such that any signal from the target material inside is overwhelmed. This problem will persist in the absence of a practical method to separate the signal of the target material from that of the container walls and atmospheric scattering.
Efforts to capture X-ray data as a function of thickness or depth generally resort to some form of tomography, in which the target material is viewed from more than one angle. In some examples, a single X-ray source and a single detector might be aligned to capture data at one angle, and then translated to capture more data at one or more additional angles in a serial sequence. In other examples, arrays of multiple X-ray sources and/or multiple detectors might be invoked to capture data at multiple angles simultaneously. A structure, such as a goniometer, might be included to facilitate reproducible translation of the source(s) and/or detector(s). In all of these cases, however, compromises are required in order to capture a three-dimensional profile. More time might be required (as in the example of a serial sequence), or there might be more exposure to radiation (as a result of multiple exposures in a serial sequence or from an array of multiple sources), or additional space might be required (to accommodate an array of sources and/or detectors), or additional complexity and expense might be imposed (for a goniometer and/or any related structure), or some combination of these factors. These compromises derive from the fact that current, conventional, state-of-the-art X-ray sensor systems are broad band and incoherent, and more significantly, they are either true CW or quasi CW with long pulse emission.
CW systems are also susceptible to increased background due to atmospheric scattering of the X-ray beam because the detector continually integrates the signal. Lacking a convenient mechanism for separating the signal of the target from radiation randomly scattered off the atmosphere, conventional X-ray systems include a random background that obscures the output.
Conventional X-ray sources such as those derived from X-ray tubes provide for a spatially divergent X-ray beam for which intensity falls off rapidly with distance. Accelerators (synchrotron for example) can provide collimated beams but with a sizeable complexity associated with such system. A collimated beam can be formed with a collimator. In the approach described here, we propose the use of an ultra-fast X-ray source such as that derived from a table-top X-ray laser and/or an ultra-fast laser initiated/driven X-ray tube (with appropriate collimating x-ray optics) which enables range gated characterization of a target material is achieved at standoff ranges.